combining two slow motion videos in split screen
You can use any effect strip (like speed control) same way like original videos. So, to get the transform effect over the speed control, select the Speed control strip and select Add -> Effect strip  ->  Transform. Or if you already have the Transform strip on original video, select the Speed control strip, then - Transform strip with Shift and then press R (Reassign Input)



Also you can group strips with Ctrl + G and you can use this group like a usual video strip: